1. Field of the Invention
The present invention relates to an electronic device including an element having a luminous material placed between electrodes and an electronic instrument using the electronic device as a display portion (indication display or indication monitor). Particularly, the present invention relates to an electronic device using a luminous material (hereinafter referred to as EL material) by which EL (Electro Luminescence) is obtained.
2. Description of the Related Art
In recent years, an electronic device (hereinafter referred to as EL display device) which uses a light-emitting device (hereinafter referred to as an EL element) using an EL phenomenon of a luminous material has been developed. Since the EL display device is a display device using the light-emitting device, a backlight as in liquid crystal display is not necessary, and further, since an angle of visibility is wide, the EL display device has attracted attention as a display portion of a portable equipment used outdoors.
There are two kinds of EL display devices, that is, a passive type (simple matrix type) and an active type (active matrix), and both types have been vigorously developed. Particularly, at present, the active matrix EL display device has attracted a great deal of attention. With respect to the EL material which becomes a luminescent layer emitting EL, there are an organic EL material and an inorganic EL material, and further, the organic EL material is classified into a low molecular (monomer) organic EL material and a high molecular (polymer) organic EL material. Especially, attention has been paid to the polymer organic EL material which is easier to handle and higher in heat resistance than the low molecular organic EL material. Incidentally, a luminous device using the organic EL material is called OLED (Organic Light Emitting Diodes) in Europe.
The active matrix EL display device is characterized in that an electric field effect transistor (hereinafter referred to as FET) is provided in each of pixels constituting a pixel portion, and an amount of electric current made to flow through an EL element is controlled by the FET. However, there has been a problem in that when electrical characteristics of the FETs vary among pixels, luminous characteristics of the EL elements provided in the respective pixels also vary.
The present invention has been made in view of the above problem and has an object to provide an electronic device in which fluctuation in the luminescence properties of EL elements among pixels is low and color reproducibility is high. Another object of the present invention is to provide a highly reliable electronic device. Further another object of the present invention is to provide an electronic instrument using the electronic device as a display portion.
Still another object of the present invention is to provide a process for reducing the manufacturing cost of the electronic device having high color reproducibility described above.
The present invention is characterized in that for the purpose of suppressing the fluctuation of electrical characteristics of FETs among pixels to a minimum, a single crystal semiconductor substrate is used as a substrate, and an electronic device is formed by using the FET formed on the single crystal semiconductor substrate. Besides, the present invention is characterized in that since the single crystal substrate having such a thickness as to enable formation of the FET does not transmit light, an EL element is formed so that a cathode is directly connected to the FET.
Further, the present invention is characterized in that a plurality of FETs are formed in one pixel, and a structure is optimized in accordance with the role of each FET, so that a highly reliable electronic device is obtained. Specifically, the present invention is characterized in that n-channel FETs are used as a switching element and an electric current controlling element, and arrangements of LDD regions of both are made different from each other.
Moreover, in the present invention, a process of forming a plurality of electronic devices from a large substrate is used to realize a reduction in the manufacturing cost of the electronic device, that is, a reduction of the cost of the electronic device. The present invention is characterized in that the process capable of using the existing liquid crystal line is employed and investment in plant and equipment is suppressed to a minimum, so that a substantial reduction in the manufacturing cost is realized.